Image forming apparatus employing non-magnetic and magnetic toner

Abstract

An image forming apparatus employs a non-magnetic developing agent and a magnetic developing agent which have been charged to opposite polarities to develop at least first and second latent images formed on an image carrier in response to image information, thus forming first and second developed images. The first and second developed images are charged to the same polarity as that of the magetic developing agent, prior to the transfer onto a transfer medium so as to provide a vivid transferred image.

What is claimed is: 
     
       1. An image forming apparatus comprising: a movable image carrier;   means for forming on said image carrier at least first and second latent images corresponding to image information;   a first developing device for developing said first latent image with non-magnetic toner charged to a predetermined polarity;   a second developing device disposed downstream of said first developing device with respect to the movement direction of said image carrier for developing said second latent image with one-component magnetic toner charged to a polarity opposite to the predetermined polarity, said second developing device including a toner carrier which is coated with a thin layer of said magnetic toner and is spaced from said image carrier to provide a gap between the surface of said image carrier and said thin layer of magnetic toner, a magnetic member disposed within said toner carrier, and means for applying an electric field across the gap to cause said magnetic toner to jump from said toner carrier onto said second latent image;   pre-transfer charging means for charging said first and second toner images developed by said non-magnetic and magnetic toners, respectively, to the same polarity as that of said magnetic toner before transfer; and   means for transferring said charged first and second toner images onto a transfer medium.   
     
     
       2. An image forming apparatus according to claim 1, wherein said first developing device contains two-component developing agent which is composed of said non-magnetic toner and magnetic particles. 
     
     
       3. An image forming apparatus according to claim 1, wherein said latent image forming means includes a first charging means for charging said image carrier to a predetermined polarity, first radiation means for irradiating said charged image carrier with first radiation corresponding to first image information, and second radiation means for irradiating said image carrier on which said first radiation has been irradiated with second radiation corresponding to second image information. 
     
     
       4. An image forming apparatus according to claim 3, wherein said first latent image is formed on said image carrier by said first radiation means, and said second latent image is formed on said image carrier by said second radiation means after said first latent image has been developed. 
     
     
       5. An image forming apparatus according to claim 3, including a second charging means for charging said image carrier to the same polarity as that of the charging conducted by said first charging means after said first radiation has been irradiated but before said second radiation is irradiated onto said image carrier. 
     
     
       6. An image forming apparatus according to claim 1, wherein said first and second toner images are in different colors. 
     
     
       7. An image forming apparatus according to claim 6, wherein said first toner image is developed by said non-magnetic toner in a chromatic color, and said second toner image is developed by said magnetic toner in black. 
     
     
       8. An image forming apparatus according to claim 3, wherein said image carrier is exposed with said first radiation at a portion corresponding to an image portion of said first image information and is exposed with said second radiation at a portion corresponding to a background portion of said second image information. 
     
     
       9. An image forming apparatus according to claim 8, wherein the first latent image formed by said first radiation is reverse developed and the second latent image formed by said second radiation is normally developed. 
     
     
       10. An image forming apparatus according to claim 5, wherein said image carrier is exposed with said first radiation at a portion corresponding to a background portion of said first image information and is exposed with said second radiation at a portion corresponding to an image portion of said second image information. 
     
     
       11. An image forming apparatus according to claim 10, wherein the first latent image formed with said first radiation is normally developed and the second latent image formed with said second radiation is reverse developed. 
     
     
       12. An image forming apparatus according to claim 5, wherein said second charging means includes a control grid in facing relationship to said image carrier. 
     
     
       13. An image forming method comprising the steps of: forming first and second latent images corresponding to image information on a movable image carrier;   developing the first latent image with a first developing device containing non-magnetic toner charged to a predetermined polarity;   developing the second latent image with a second developing device containing magnetic toner charged to a polarity opposite to the predetermined polarity by spacing a thin layer of the magnetic toner formed on a toner carrier from the surface of said image carrier by a gap and applying an electric field across the gap to cause the magnetic toner to jump from the toner carrier to the second latent image, wherein the second developing device is located downstream of the first developing device with respect to the moving direction of the image carrier;   charging the first and second developed images developed by the non-magnetic and magnetic toners, respectively, to the same polarity as that of the magnetic toner before transfer; and   transferring the charged first and second developed images onto a transfer medium.   
     
     
       14. An image forming method according to claim 13, wherein the non-magnetic toner is mixed with magnetic carrier particles. 
     
     
       15. An image forming method according to claim 13, wherein said image forming step includes a first charging step for charging the image carrier to a predetermined polarity, a first radiation step for irradiating the charged image carrier with first radiation corresponding to first image information, and a second radiation step for irradiating the image carrier on which the first radiation has been irradiated with second radiation corresponding to second image information. 
     
     
       16. An image forming method according to claim 15, wherein the second radiation step is performed after the first latent image has been developed. 
     
     
       17. An image forming method according to claim 16, wherein said image forming step further comprises a second charging step for charging the image carrier to the same polarity as that of the first charging step after the first radiation step but before the second radiation step. 
     
     
       18. An image forming method according to claim 13, wherein the first latent image is developed by the non-magnetic toner in a chromatic color, and the second latent image is developed by the magnetic toner in block. 
     
     
       19. An image forming method according to claim 15, wherein the image carrier is exposed with the first radiation at a portion of the first image information and is exposed with the second radiation at a portion corresponding to a background portion of the second image information. 
     
     
       20. An image forming method according to claim 19, wherein the first latent image formed with the first radiation is reverse developed and the second latent image formed with the second radiation is normally developed. 
     
     
       21. An image forming method according to claim 17, wherein the image carrier is exposed with the first radiation at a portion corresponding to a background portion of the first image information and is exposed with the second radiation at a portion corresponding to an image portion of the second image information. 
     
     
       22. An image forming method according to claim 21, wherein the first latent image formed with the first radiation is normally developed and the said second latent image formed with the second radiation is reverse developed. 
     
     
       23. An image forming apparatus according to claim 1, wherein said applying means applies an alternating electric field. 
     
     
       24. An image forming apparatus according to claim 1, wherein said charged first and second toner images are transferred at the same time onto the transfer medium. 
     
     
       25. A method according to claim 13, wherein an alternating electric field is applied to effect development of the second latent image. 
     
     
       26. A method according to claim 13, wherein the charged first and second toner images are transferred at the same time onto the transfer medium.